1. Field of the Invention
The present invention relates generally to a fluid filled type vibration damping device capable of providing vibration damping effect based on the flow action of a fluid sealed therein, and more particularly to a fluid filled type vibration damping device having an elastic rubber film disposed within a fluid chamber sealed with the fluid, and capable of changing its damping properties by changing the state of the elastic rubber film.
2. Description of the Related Art
Fluid filled type vibration damping devices designed to produce vibration damping effect on the basis of resonance action or other fluid action of a fluid sealed in the interior are known as one type of vibration damping device such as a vibration damped coupling or vibration damped support designed for installation between components making up a vibration transmission system. In such a fluid filled type vibration damping device, typically, a first metal mounting member is disposed on the side of a first opening of a second metal mounting member of tubular shape, the first mounting member and the second mounting member are elastically connected by a main rubber elastic body with one opening of the second mounting member being sealed off fluid-tightly, while the other opening of the second mounting member is sealed off fluid-tightly by a flexible film, thereby forming between the main rubber elastic body and the flexible film a fluid chamber in which a non-compressible fluid is sealed. A partition member supported by the second mounting member is disposed between the main rubber elastic body and the flexible film, thereby dividing the fluid chamber to form to either side of the partition member a pressure-receiving chamber whose wall is partially constituted by the main rubber elastic body and an equilibrium chamber whose wall is partially constituted by the flexible film, with the two chambers communicating with each other through an orifice passage.
Such a fluid filled type vibration damping device is able to exhibit excellent vibration damping effect with respect to low frequency vibration, such as engine shake, by means of flow action of the non-compressible fluid through the orifice passage.
However, although this type of fluid filled vibration damping device is able to exhibit excellent vibration damping effect with respect to the predetermined frequency range to which the orifice passage is tuned, like engine shake, it was difficult for this type of vibration damping device to exhibit sufficient damping effect with respect to vibration such as idling vibration or booming noise, in a frequency range higher than the tuning frequency of the orifice passage, since the orifice passage is held in substantially closed state upon input of these high frequency vibration, whereby a dynamic spring constant of the device will become higher.
To cope with this problem, an improved fluid-filled type vibration damping device was disclosed in U.S. Pat. No. 6,082,718, wherein a deformable elastic rubber film is disposed such that on one side of the elastic rubber film, there is formed a primary fluid chamber whose wall is partially defined by a first surface of the elastic rubber film, and on the other side of the elastic rubber film, there is formed a pressure operating chamber. The other surface of the elastic rubber film faces to the pressure operating chamber, and a pressure in the pressure operating chamber is controlled from the outside. This makes it possible to change vibration damping characteristics of the device.
In the fluid filled type vibration damping device disclosed in U.S. Pat. No. 6,082,718, upon input of vibration having the same frequency to which the orifice passage is tuned, a negative pressure is applied to the pressure operating chamber in order to attract the elastic rubber film to an attraction face of the pressure operating chamber, thereby limiting deformation of the elastic rubber film. With this arrangement, a sufficient amount of fluid flowing through the orifice passage is obtained, whereby the vibration damping device is able to exhibit high damping effect. On the other hand, upon input of vibration having a frequency higher than the tuning frequency of the orifice passage, the pressure operating chamber is held in the atmospheric pressure. With this arrangement, the elastic rubber film is allowed to deform, whereby pressure fluctuation in the primary fluid chamber due to the input of vibration can be absorbed by means of deformation of the elastic rubber film.
In the fluid filled type vibration damping device disclosed in U.S. Pat. No. 6,082,718, it is desirable to stably and quickly change the vibration damping characteristics in order to ensure excellent vibration damping effect. More specifically, it is required to quickly change the operating state of the vibration damping device between one state where the deformation of the elastic rubber film is limited by applying negative pressure to the pressure operating chamber and the other state where the deformation of the elastic rubber film is permitted by keeping the pressure operating chamber in the atmospheric pressure.
However, in the vibration damping device of construction according to U.S. Pat. No. 6,082,718, a tiny gap is formed between elastic rubber film and the attraction face of the pressure operating chamber. Accordingly, even if the negative pressure is applied to the pressure operating chamber, the deformation of the elastic rubber film is permitted at the portion where the tiny gap is formed. With this drawback, the pressure fluctuation generated in the primary fluid chamber is absorbed by means of deformation of the elastic rubber film. It may be difficult for the disclosed vibration damping device to obtain sufficient amount of fluid flowing through the orifice passage. Thus, the vibration damping device is less likely to exhibit sufficient vibration damping effect.
In addition, the elastic rubber film is retracted first at a portion located close to a negative pressure suction hole. This portion of the elastic rubber film is likely to fully close the negative pressure suction hole at the earlier stage of the negative pressure applying operation. As a result, the attraction of the elastic rubber film is not sufficiently completed, so that a large air trapped may be formed between the elastic rubber film and the attraction face. This trapped air will permit the deformation of the elastic rubber film, like the above described tiny gap, the desired high vibration damping effect might not be exhibited sufficiently.
As will be understood from the foregoing description, the fluid-filled vibration damping device disclosed in U.S. Pat. No. 6,082,718 is disable to stably switch the absorption state and non-absorption state of the elastic rubber film, since the deformation of the elastic rubber film is not stable. Therefore, it is difficult for the disclosed vibration damping device to switch its vibration damping properties quickly and stably.